(377l) Theoretical Detection of a HDA-Lda Liquid-Liquid Transition Phase for Water Using Molecular Simulation

Recent work concerning liquid-phase water molecular simulation and theory suggests that some anomalies observed in thermodynamic properties may be related to a transition between two liquid states, the so called HDA-LDA transition (acronym for high density to low density). As an example, the density maximum at 4 °C and the minimum in the isothermal compressibility at 46 °C are believed to be related to the existence of this phase transition. In this work we present the results of molecular simulations using the TIP4P model for water at very low temperatures and high pressures. Molecular simulations were performed with the software GROMACS 5.1.4 and ad-hoc post-processing codes implemented to analyze the results. Pressures from 600 MPa to 1400 MPa and temperatures from 263 K to 113 K were analyzed. While these conditions are only experimentally accessible by adding salts, molecular simulations were conducted without considering any added salt.

The analysis of radial distribution function showed the formation of a new hydration shell, with a corresponding increase in the coordination number, when temperature decreases. The density profile shows a discontinuity in the derivative of the density as a function of temperature. These facts show that the HDA-LDA can indeed be predicted by molecular simulation, and that it constitutes a second order transition.